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City of Morgan Hill
Sanitary Sewer Flow Monitoring and Inflow/Infiltration Study
12-0248 AEG CofMorganHill FM Rpt.docx
Page 23 of 46
Flow Monitoring: Peak Measured Flows and Pipeline Capacity Analysis
Peak measured flows and the corresponding flow levels (depths) are important to understand the
capacity limitations of a collection system. The peak flows and flow levels reported are from the peak
measurements as taken across the entirety of the flow monitoring period. Peak flows and levels may
not correspond to a rainfall event, but instead may be caused due to blockages, grease or roots that
cause a backflow condition.
Two key capacity analysis terms are defined as follows:
Peaking Factor:
Peaking factor is defined as the peak measured flow divided by the average
dry weather flow (ADWF). A peaking factor threshold value of 3.0 is commonly used for
sanitary sewer design.
d
/
D
Ratio:
The
d
/
D
ratio is the peak measured depth of flow (
d
) divided by the pipe diameter
(
D
). A
d
/
D
ratio of 0.75 is a common maximum threshold value used for pipe design. The
d
/
D
ratio for each site was computed based on the maximum depth of flow from the flow
monitoring study.
Table 9 summarizes the peak recorded flows, levels,
d
/
D
ratios, and peaking factors per site during
the flow monitoring period. Capacity analysis data is presented on a site-by-site basis and represents
the hydraulic conditions only at the site locations; hydraulic conditions in other areas of the collection
system will differ.
Table 9. Capacity Analysis Summary
Metering
Site
ADWF
(mgd)
Peak
Measured
Flow
(mgd)
Peaking
Factor
Diameter
(in)
Peak
Level
(in)
Peak
d
/
D
Ratio
Level
Surcharged
above
Crown (ft)
Site 1
0.19
0.56
2.93
18
3.73
0.21
-
Site 2
0.36
0.88
2.45
19.75
7.94
0.40
-
Site 3
0.32
0.93
2.95
15
13.13
0.88
-
Site 4
0.21
0.53
2.49
12
6.04
0.50
-
Site 5
1.15
2.29
1.98
24
15.95
0.66
-
Site 6
0.35
0.91
2.60
17.75
12.52
0.71
-
Site 7
0.24
0.77
3.17
21
6.82
0.32
-
Site 8
0.16
0.43
2.73
17.5
6.76
0.39
-
Site 9
0.13
0.41
3.03
15
6.36
0.42
-
Site 10
0.12
0.31
2.45
11.75
4.57
0.39
-
The following capacity analysis results are noted:
Peaking Factor:
Sites 7 and 9 had peaking factors that slightly exceeded typical design
threshold limits for the ratio of peak flow to average dry weather flow.
d
/
D
Ratio:
Only Site 3 had a
d
/
D
ratio that exceeded the common design threshold for
d
/
D
ratio. None of the flow monitoring sites reached surcharge conditions.